Author Topic: Good forums to help fix electronics? (Read 4191 times)

tjohnson · « **on:** 15:11, 06 April 20 »

Massively off topic for Amstrad but thought someone might know some good forums where I could seek help on fixing a double cassette deck.

The original issue I started to diagnose was the recording deck played fine but the recording would intermiitently distort on one channel. I have access to a scope and observed the bias signal voltage on the affected channel changing very erratically. However in my effort to diagnose I seem to have introduced a playback issue on that deck which plays back at massive amp and hugely distorted however it still records. I think it may be in a multiplexer IC which takes the inputs for deck A and B then outputs them. Deck A seems to play back more or less ok. Same IC and outputs but seperate inputs.

I have the service manual and it has a very good circuit and block diagrams but with my little experience struggling to pinpoint this new fault but being a hobbist and complete amateur I'm very much trying to learn as I go along.

Bryce · « **Reply #1 on:** 16:45, 06 April 20 »

Send me the schematics and I'll see if I can help.

Bryce.

tjohnson · « **Reply #2 on:** 23:56, 06 April 20 »

Hi Bryce, thanks for the offer of some help.

I've attached the main schematic and a block diagram. The original issue appeared to be a bias issue, i hooked up the scope to capacitors C130 and C230 and could see on record the bias on the right channel C230 bouncing up and down alot in voltage. Anyway I was trying to determine the cause of this when I then noticed the playback on deck B had gone completly distorted and pushing out massive amplification where previously it had worked fine. I've attached a sound clip with playback on deck B and then playback on deck A which still seems to works. I was suspecting an issue in the multiplexer IC301 on in that region of the circuit as playback A still works, however I'm really clutching at straws here and could really do with some guidance on where might be suitable places to check and understanding the circuit digram, it would be really appreciated.
Trevor

Bryce · « **Reply #3 on:** 10:11, 07 April 20 »

Ok. Obviously the first culprit would be a bad electrolytic capacitor, but I assume you've already given the board a visual inspection. The next step would be to disconnect the heads and connect a signal generator to each channel at the head connector and compare the signals after the bias traps (L104 and L204) with a scope.

Bryce.

tjohnson · « **Reply #4 on:** 14:54, 07 April 20 »

I should say that playback on deck B was fine until I started probing around, I thought I was very careful not to short anything out but I'm not exactly sure when it went wrong but I think it was after I connected the scope up to the legs of C129 and C229.
I had previously checked playback by playing a sinewave 1000hz into the inputs, recording and then playing back. I hooked the scope up to the legs of C135 and C235 to see the tone and it was fine on both deck A and B, now B is fubar.

On a further visual inspection of the board I noticed that the large 4700uf cap on the powerboard C909 has a small bulge in it. I had previously dropped the board out and resoldered a few points that looked a little iffy, nothing else looks bad from what I can see. Annoyingly I don't have a spare but could order one in, fingers crossed it arrives in all this madness. Unfortunately I don't have a function generator and getting one at the moment could be a challenge!

Bryce · « **Reply #5 on:** 17:38, 07 April 20 »

An MP3 player with a 10kHz sinewave recording can be used instead of a signal generator.

Bryce.

tjohnson · « **Reply #6 on:** 17:51, 07 April 20 »

Thanks, I had that thought after I wrote my response. Hopefully I'll be able update tomorrow on the results.

tjohnson · « **Reply #7 on:** 01:49, 08 April 20 »

Right so I made a little cable up and hooked it up to CN041 on the board for deck B.
I set the test tone to 1khz I used spotify to play audio test tones 1khz at 0db, starting with vol turned down.
I hooked the scope up to c130 which comes off L104 (I check both C130 and C230 and the same end result)
I used a set of headphones to listen.
Without pressing play I heard nothing.
Pressing play I heard the test tone ok without the same level of distortion however as I turned the volume up distortion was heard although not quite the same and I started to pickup noise on the scope.
When playing the test tone at normal audible volume there was no additional noise at c130 than what I observed without playing.
I then hooked up the head back onto the board at CN041 and played the tape and observed noise at c130 (see picture) and the horrible playback sound through the headphones.
I also recorded, obviously this didn't record to the tape as the head wasn't connected, and observed what I assume is the bias signal at 107khz (see picture).
So I'm a bit perplexed as to what the issue could be and the source of the noise.

Sorry about the bullet point, the forum put everything into one big Paragraph.

tjohnson · « **Reply #8 on:** 23:43, 08 April 20 »

I have spent ages trying to work out what is happening and have now have an interesting update.

I then fed in the signal from deck A on one channel into the B connector CN041 on the board again and played from deck A. I heard nothing but upon connecting deck B back up it now plays perfectly fine without any distortion but recording is now not working where it was before.

So the issue is now reversed, Deck B is now playing without distortion where it wasn't before but now doesn't records silence where it recorded before.

The board has alot of ICs and a main processor, I can see in the block diagram something called record mute, I'm wondering if this is some sort of record muting issue. I'm not quite sure how that's suppose to work but maybe it should mute on playback but not on record and wasn't muting before on playback so putting a high voltage signal in during the playback hence the massive distortion but is now stuck in mute.....atleast now I have more of a chance to tracking the issue down I think as I can monitor for the record signal....the mystery continues.

Bryce · « **Reply #9 on:** 10:58, 09 April 20 »

If this is an expensive piece of kit and you seriously want it back working (and both of those seem to be the case), then the first thing I'd do is replace every electrolytic capacitor. When the more complicated parts such as multiplexers, Op-AMP's, CPU's etc have issues, you usually loose an entire channel or the entire device is dead. Distortion means it's working, but something is outside the specs. On really old valve radios, this could also be a resistor drift issue, but not here, the only thing that goes bad are the electrolytics.

Bryce.

tjohnson · « **Reply #10 on:** 13:25, 09 April 20 »

Its now a bit of a mission to fix it! I think I'll start with the power supply board which has most of the big caps on and the large 4700uf which is on the 12v line shows some signs of bulging.
I did some more testing yesterday and after the rather unusual change in the issue where playback now works on deck B but record doesn't , i.e. the issue is reversed I can pickup the audio signal on the record path just before the pre-equalisation amp input (IC306) but not on the outputs. On record I see the signal rise a bit approx 100mv when recording but there is no audio it stays flat.
I feel this is a bit like looking for a needle in a haystack with only half an eye open!

Bryce · « **Reply #11 on:** 15:40, 09 April 20 »

How much ripple/noise is on the power rails? Observe them during the different functions.

Bryce.

tjohnson · « **Reply #12 on:** 22:22, 09 April 20 »

I'll give that a try, just gotta work out how, looks like IC306 is driven off the +8v and -8v lines. I'm wondering about TR317, that appears to be involved in muting record. from the circuit diagram it show the base hooked up to GND, the emitter is on the rec mute from the main processor and then the collector is hooked up to the base of 2 transistors in the rec output, so I'm assuming that this TR317 transistor is acting as a switch to mute the recording output via the two other transistors but I don't really know enough to fully understand how this circuit works.
Trevor

tjohnson · « **Reply #13 on:** 17:03, 16 April 20 »

not much of an update but I recapped the power board, no change, this thing still doesn't record.
My next area of investigation is here. I could pickup the recording signal in the area before the pre-equalisation amp marked in green but not on the outputs. There is a transistor TR317 involved in record muting and a multiplexer that sets equalisation depending on tape type so will look at these.
This is probably a question for bryce, on the transistor it is a DTA114TS (PNP) which appears to have a built in 10k resistor on the base, if measuring with DDM positive on the base then meeter OL when neg to Emitter and Collector but forward volatge in reverse? I couldn't quite work out the datasheet if there is a resistor from base to emmitter too and whether this would affect the reading?

Bryce · « **Reply #14 on:** 17:52, 16 April 20 »

Those are relatively difficult to measure. You may be able to test it with the diode tester on the multimeter, at least between Base and Collector (with black probe to base), but the Base - Emitter diode won't be testable due to the series resistor. Anyway, those three transistors are the mute circuitry, so if you remove TR108 and TR208 the mute is removed and you should get a recording signal (if that was the issue).

Btw: I hope you've seen that C125 and C225 are bi-polar electrolytics! So if you've swapped them, you better have used BP Electrolytics too, otherwise you'll have built a smoke machine.

Bryce.

tjohnson · « **Reply #15 on:** 17:56, 16 April 20 »

cheers I haven't changed those caps, I only did the power board. I can't believe those caps have both failed all of a sudden together, good idea on trying to remove those transistors, I'll take them out and see if that makes a difference to recording.

tjohnson · « **Reply #16 on:** 16:24, 17 April 20 »

I pulled transistors TR108 and TR208 and that has made no difference still recording silence. Need to get the scope back out now and have another look at what is going on again.

Bryce · « **Reply #17 on:** 17:32, 17 April 20 »

Then the problem is most likely IC305 or one of the control signals going to that chip.

Bryce.

tjohnson · « **Reply #18 on:** 20:35, 17 April 20 »

That is my concern

, I'm really hoping the issue is going to be IC307, that chip provides the parameters to IC305 so if anything is bust hopefully it's that chip as it seems to be an available part. On IC307 the only control connected is the ground on pin 5. All the other control pins appear to be NC on the circuit.

Bryce · « **Reply #19 on:** 20:42, 17 April 20 »

CXA = Sony part. so a pain in the arse to get in many circumstances. If that's fried it will be difficult to find, but I would be suprised if it was, it's not a high current or high load part, so why would it fail?

Bryce.

tjohnson · « **Reply #20 on:** 21:24, 17 April 20 »

Ok was checking IC307 and have found D308 has 8.2v on one side and 3.5v on the other which doesn't sound right at all I don't thnk it should be dropping over 4v. On the negative side D307 I see -8v and -7.2 so the expected drop. That lower measurements is on the cathode side, it feeds both IC306 and IC307. Neither IC seems hot.

tjohnson · « **Reply #21 on:** 02:47, 18 April 20 »

Oh man my head hurts. Diode D308 had failed. D307 was ok. So I've replace them both anyway. now the awful noise on playback on deck B has returned, recording partially works but on left channel only, so I'm back to before but with the loss of one channel while recording. Deck A is still playing. As I say my head hurts time for bed!

Bryce · « **Reply #22 on:** 10:18, 18 April 20 »

If D308 has failed, the IC307 most likely has an internal short circuit. Disconnect one end of D308 and measure the current. It's a low power multiplexer, it shouldn't pulling more than 300mA which is what the 1N4148 can handle.

Bryce.

tjohnson · « **Reply #23 on:** 10:53, 18 April 20 »

Sleeping on it has helped I think. I reckon D308 is a bit of a red herring. Before I connected my phone to the header for deck B I could record both channels but play back was completely dissorted. After doing so playback was fine but no recording. D308 is on the left channel which is now recording again so it seems connecting my phone somehow blew the diode but now I've lost recording on right channel. This probably explains why I'm now almost back to the original issue. I definitely remenber checking the voltages across that diode before and they were fine. I'm now back to thinking that muting transistor tr317 is the fault in that it's not muting when in playback or the control ic that sends the signal to mute. No idea why right channel has gone, I wouldn't think a phone could put out the voltage or current to damage something but maybe it has since the head itself is just a coil isn't it? So I'm going to try to replace the muting tr317, could I use a normal pnp with a 10k resistor soldered to the base, that is what the service manual suggests it is. I don't have this type and it seems no longer produced.

Bryce · « **Reply #24 on:** 10:55, 18 April 20 »

But you already said you removed both muting transistors and that didn't change anything?

Bryce.

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Author Topic: Good forums to help fix electronics? (Read 4191 times)